An interrupt is any action which causes the temporary cessation of execution by a processor of a particular process so that the processor can execute an interrupt process. Such action is typically initiated in processor circuits by peripheral devices requesting the processor to interrupt execution of a main process to execute an interrupt process associated with that particular peripheral device.
Known processors have been designed with a capacity to support a fixed number of interrupt priority levels. It may be necessary for a processor to support more than one interrupt since different interrupt requests require different interrupt processes. Not only do processors need to be able to support multiple interrupt requests, but they also need to be able to prioritise such requests should they occur simultaneously.
However, the number of interrupt priority levels which can be supported may be restricted by the processor. Typically a processor may support only two interrupt priority levels. Although processors may be adapted to support additional levels of interrupt, such adaptation is complex and it may be necessary to modify the instruction set of the processor and also to provide additional registers.